We present the latest update of the European Southern Observatory's Very Large Telescope interferometer (VLTI). The operations of VLTI have greatly improved in the past years: reduction of the execution time; better offering of telescopes configurations; improvements on AMBER limiting magnitudes; study of polarization effects and control for single mode fibres; fringe tracking real time data, etc. We present some of these improvements and also quantify the operational improvements using a performance metric. We take the opportunity of the first decade of operations to reflect on the VLTI community which is analyzed quantitatively and qualitatively. Finally, we present briefly the preparatory work for the arrival of the second generation instruments GRAVITY and MATISSE.
MATISSE is the mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This second generation interferometry instrument will open new avenues in the exploration of our Universe. Mid-infrared interferometry with MATISSE will allow significant advances in various fundamental research fields: studies of disks around young stellar objects where planets form and evolve, surface structures and mass loss of stars in late evolutionary stages, and the environments of black holes in active galactic nuclei. MATISSE is a unique instrument. As a first breakthrough it will enlarge the spectral domain used by optical interferometry by offering the L & M bands in addition to the N band, opening a wide wavelength domain, ranging from 2.8 to 13 μm on angular scales of 3 mas (L/M band) / 10 mas (N band). As a second breakthrough, it will allow mid-infrared imaging – closure-phase aperture-synthesis imaging – with up to four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. MATISSE will offer various ranges of spectral resolution between R~30 to ~5000. In this article, we present some of the main science objectives that have driven the instrument design. We introduce the physical concept of MATISSE including a description of the signal on the detectors and an evaluation of the expected performance and discuss the project status. The operations concept will be detailed in a more specific future article, illustrating the observing templates operating the instrument, the data reduction and analysis, and the image reconstruction software.
The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8-m Unit Telescopes (UT) and the four 1.8-m Auxiliary Telescopes (AT) of the Paranal Observatory. After the first fringes obtained in 2011 with the commissioning instrument VINCI and with siderostats, the VLTI has seen an important number of systems upgrades, paving the path towards reaching the infrastructure level and scientific results it had been designed for. The current status of the VLTI operation all year round with up to four telescopes simultaneously and real imaging capability demonstrates the powerful interferometric infrastructure that has been delivered to the astronomical community. Reaching today’s level of robustness and operability of the VLTI has been a long journey, with a lot of lessons learned and gained experience. In 2007, the Paranal Observatory recognized the need for a global system approach for the VLTI, and a dedicated system engineering team was set to analyse the status of the interferometer, identify weak points and area where performances were not met, propose and apply solutions. The gains of this specific effort can be found today in the very good operability level with faster observations executions, in the decreased downtime, in the improved performances, and in the better reliability of the different systems. We will present an historical summary of the system engineering effort done at the VLTI, showing the strategy used, and the implemented upgrades and technical solutions. Improvements in terms of scientific data quality will be highlighted when possible. We will conclude on the legacy of the VLTI system engineering effort, for the VLTI and for future systems.
The New Adaptive Optics Module for Interferometry (NAOMI)1 is the future low order adaptive optics system to be developed for and installed at the ESO 1.8 m Auxiliary Telescopes (ATs). The four ATs2 are designed for interferometry which they are essentially dedicated for. Currently the AT’s are equipped with a fast, visible tip-tilt sensor called STRAP3 (System for Tip/tilt Removal with Avalanche Photodiodes), and the corrections are applied through a tip-tilt mirror. The goal is to equip all four ATs with a low-order Shack-Hartmann system operating in the visible for the VLTI dual feed light beams in place of the current tip-tilt correction. Because of the limited size of the ATs (1.8m diameter), a low-order system will be sufficient. The goal is to concentrate the energy into a coherent core and to make the encircled energy (into the single mode fibers) stable and less dependent on the atmospheric conditions in order to increase the sensitivity of the interferometric instruments. The system will use the ESO real time computer platform Sparta-light as the baseline. This paper presents the preliminary design concept and outlines the benefits to current and future VLTI instruments.
The ESO Very Large Telescope Interferometer (VLTI) using the Unit Telescope (UT) was strongly affected by vibrations since the first observations. Investigation by ESO on that subject had started in 2007, with a considerable effort since mid 2008. An important number of investigations on various sub-systems (On telescope: Guiding, Passive supports, Train Coude, insulation of electronics cabinets; On Instruments: dedicated campaign on each instruments with a special attention on the ones equipped with Close Cycle Cooler) were realized. Vibrations were not only recorded and analyzed using the usual accelerometers but also using on use sub-systems as InfRared Image Sensor (IRIS) and Multiple Applications Curvature Adaptive Optics (MACAO) and using a specific tool developed for vibrations measurements Mirror vibrAtion Metrology systeM for the Unit Telescope (MAMMUT). Those tools and systems have been used in order to improve the knowledge on telescope by finding sources. The sources whenever it was possible were damped. As known for years, instruments are still the principal sources of vibrations, for the majority of the UT. A special test in which 2 UTs instruments were completely shut down was realized to determine the minimum Optical Path Length (OPL) achievable. Vibrations is now a part of the instruments interface document and during the installation of any new instrument (KMOS) or system (AOF) a test campaign is realized. As a result some modifications (damping of CCC) can be asked in case of non-compliance. To ensure good operational conditions, levels of vibrations are regularly recorded to control any environmental change.
The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8-m Unit Telescopes (UT) and
the four 1.8-m Auxiliary Telescopes (AT) of the Paranal Observatory located in the Atacama Desert in
northern Chile. The two VLTI instruments, MIDI and AMBER deliver regular scientific results. In parallel to the
operation, the instruments developments are pursued, and new modes are studied and commissioned to offer
a wider range of scientific possibilities to the community and increase sensitivity. New configurations of the
ATs have been offered and are frequently discussed with the science users of the VLTI and implemented to
optimize the scientific return. The PRIMA instrument, bringing astrometry capability to the VLTI and phase
referencing to the instruments is being commissioned. The visitor instrument PIONIER is now fully operational
and bringing imaging capability to the VLTI.
The current status of the VLTI is described with successes and scientific results, and prospects on future
evolution are presented.
MATISSE is a mid-infrared spectro-interferometer combining the beams of up to four Unit Telescopes or Auxiliary
Telescopes of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory.
MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. New characteristics present in
MATISSE will give access to the mapping and the distribution of the material, the gas and essentially the dust, in the
circumstellar environments by using the mid-infrared band coverage extended to L, M and N spectral bands. The four
beam combination of MATISSE provides an efficient uv-coverage: 6 visibility points are measured in one set and 4
closure phase relations which can provide aperture synthesis images in the mid-infrared spectral regime.
We give an overview of the instrument including the expected performances and a view of the Science Case. We present
how the instrument would be operated. The project involves the collaborations of several agencies and institutes: the
Observatoire de la Côte d’Azur of Nice and the INSU-CNRS in Paris, the Max Planck Institut für Astronomie of
Heidelberg; the University of Leiden and the NOVA-ASTRON Institute of Dwingeloo, the Max Planck Institut für
Radioastronomie of Bonn, the Institut für Theoretische Physik und Astrophysik of Kiel, the Vienna University and the
The Phase Referenced Imaging and Micro Arcsecond Astrometry (PRIMA) facility for the Very Large Telescope
Interferometer (VLTI), is being installed and tested in the observatory of Paranal. Since January 2011 the
integration and individual testing of the different subsystem has come to a necessary minimum. At the same
time the astrometric commissioning phase has begun.
In this contribution we give an update on the status of the facility and present some highlights and difficulties
on our way from first dual-feed fringe detection to first astrometric measurements. We focus on technical and
operational aspects. In particular, within the context of the latter we are going to present a modified mode of
operation that scans across the fringes. We will show that this mode, originally only intended for calibration
purposes, facilitates the detection of dual-fringes.
The visitor instrument PIONIER provides VLTI with improved imaging capabilities and sensitivity. The in-
strument started routinely delivering scientic data in November 2010, that is less than 12 months after being
approved by the ESO Science and Technical Committee. We recall the challenges that had to be tackled to design, built and commission PIONIER. We summarize the typical performances and some astrophysical results obtained so far. We conclude this paper by summarizing lessons learned.
The Phase Referenced Imaging and Micro Arcsecond Astrometry (PRIMA) facility for the Very Large Telescope
Interferometer (VLTI), is being installed and tested in the observatory of Paranal. Most of the tests have been
concentrated on the characterization of the Fringe Sensor Unit (FSU) and on the automation of the fringe
tracking in preparation of dual-field observations. The status of the facility, an analysis of the FSU performance
and the first attempts towards dual-field observations will be presented in this paper. In the FSU, the phase
information is spatially encoded into four independent combined beams (ABCD) and the group delay comes from
their spectral dispersion over 5 spectral channels covering the K-band. During fringe tracking the state machine
of the optical path difference controller is driven by the Signal to Noise Ratio (SNR) derived from the 4 ABCD
measurements. We will describe the strategy used to define SNR thresholds depending on the star magnitude
for automatically detecting and locking the fringes. Further, the SNR as well as the phase delay measurements
are affected by differential effects occurring between the four beams. We will shortly discuss the contributions
of these effects on the measured phase and SNR noises. We will also assess the sensitivity of the group delay
linearity to various instrumental parameters and discuss the corresponding calibration procedures. Finally we
will describe how these calibrations and detection thresholds are being automated to make PRIMA as much as
possible a user-friendly and efficient facility.
GRAVITY is an adaptive optics assisted Beam Combiner for the second generation VLTI instrumentation. The
instrument will provide high-precision narrow-angle astrometry and phase-referenced interferometric imaging in the
astronomical K-band for faint objects. We describe the wide range of science that will be tackled with this instrument,
highlighting the unique capabilities of the VLTI in combination with GRAVITY. The most prominent goal is to observe
highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky
Way. We present the preliminary design that fulfils the requirements that follow from the key science drivers: It includes
an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near-infrared wavefrontsensing
adaptive optics; fringe-tracking on secondary sources within the field of view of the VLTI and a novel metrology
concept. Simulations show that 10 μas astrometry within few minutes is feasible for a source with a magnitude of
mK = 15 like Sgr A*, given the availability of suitable phase reference sources (mK = 10). Using the same setup, imaging of mK = 18 stellar sources in the interferometric field of view is possible, assuming a full night of observations and the corresponding UV coverage of the VLTI.
The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8-m Unit Telescopes (UT) and the four
1.8-m Auxiliary Telescopes (AT) of the Paranal Observatory located in the Atacama Desert in northern Chile. The two
VLTI instruments, MIDI and AMBER deliver regular scientific results. In parallel to the operation, the instruments
developments are pursued, and new modes are studied and commissioned to offer a wider range of scientific possibilities
to the community. New configurations of the ATs array are discussed with the science users of the VLTI and
implemented to optimize the scientific return. The monitoring and improvement of the different systems of the VLTI is a
continuous work. The PRIMA instrument, bringing astrometry capability to the VLTI and phase referencing to the
instruments has been successfully installed and the commissioning is ongoing. The possibility for visiting instruments
has been opened to the VLTI facility.
The ESO Very Large Telescope Interferometer (VLTI) offers the unique access to the combination of the four 8-meter
Unit Telescopes (UT) of Cerro Paranal. The quality of the scientific observations in interferometric mode is strongly
related to the stability of the optical path difference (OPD) between the telescopes. Vibrations at the level of the
telescopes and affecting the mirrors were shown to be an important source of perturbation for the OPD.
ESO has thus started an important effort on the UTs and VLTI to tackle this effect. Active controls based on
accelerometers and phase measurements have been developed to provide real-time correction of the variation of OPD
introduced by vibrations. Systematic studies and measurement of the sources of vibration (instruments, wind, telescope
altitude, ...) have been performed. Solutions to reduce the vibrations via design modification and/or new operation
configurations are studied and implemented. To ensure good operational conditions, the levels of vibrations are regularly
monitored to control any environmental change. This document will describe the modifications implemented and
foreseen and give a status of the VLTI-UT vibrations evolution.
PIONIER is a 4-telescope visitor instrument for the VLTI, planned to
see its first fringes in 2010. It combines four ATs or four UTs
using a pairwise ABCD integrated optics combiner that can also be
used in scanning mode. It provides low spectral resolution in H and K band. PIONIER is designed for
imaging with a specific emphasis on fast fringe recording to allow
closure-phases and visibilities to be precisely measured. In
this work we provide the detailed description of the instrument and
present its updated status.
The first phase of the VLTI is now almost completed with 8 available telescopes, 2 science instruments and
complementary facilities (in particular a fringe tracker and angle tracker). The next step will be the integration of
PRIMA and the development of the second generation instruments for which a thorough understanding of the
performance that can provided by the VLTI facility is required. This is the basic purpose of this ongoing complementary
characterization. The rationale of this activity is recalled and the methods used as well as the results obtained so far
The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8 m Unit Telescopes (UT) and the four
1.8 m Auxiliary Telescopes (AT) of the Paranal Observatory located in the Atacama Desert in northern Chile. The fourth
AT has been delivered to operation in December 2006, increasing the flexibility and simultaneous baselines access of the
VLTI. Regular science operations are now carried on with the two VLTI instruments, AMBER and MIDI. The FINITO
fringe tracker is now used for both visitor and service observations with ATs and will be offered on UTs in October
2008, bringing thus the fringe tracking facility to VLTI instruments. In parallel to science observations, technical periods
are also dedicated to the characterization of the VLTI environment, upgrades of the existing systems, and development
of new facilities. We will describe the current status of the VLTI and prospects on future evolution.
PRIMA, the Phase-Referenced Imaging and Micro-arcsecond Astrometry facility for the Very Large Telescope Interferometer, is now nearing the end of its manufacturing phase. An intensive test period of the various sub-systems (star separators, fringe sensor units and incremental metrology) and of their interactions in the global system will start in Garching as soon as they are delivered. The status and performances of the individual sub-systems are presented in this paper as well as the proposed observation and calibration strategy to reach the challenging goal of high-accuracy differential astrometry at 10 μas level.
The ESO Very Large Telescope Interferometer (VLTI) is the first general-user interferometer that offers near- and mid-infrared long-baseline interferometric observations in service and visitor mode to the whole astronomical community. Over the last two years, the VLTI has moved into its regular science operation mode with the two science instruments, MIDI and AMBER, both on all four 8m Unit Telescopes and the first three 1.8m Auxiliary Telescopes. We are currently devoting up to half of the available time for science, the rest is used for characterization and improvement of the existing system, plus additional installations. Since the first fringes with the VLTI on a star were obtained on March 17, 2001, there have been five years of scientific observations, with the different instruments, different telescopes and baselines. These observations have led so far to more than 40 refereed publications. We describe the current status of the VLTI and give an outlook for its near future.
Darwin is one of the most challenging space projects ever considered by the European Space Agency (ESA). Its principal objectives are to detect Earth-like planets around nearby stars and to characterise their atmospheres. Darwin is conceived as a space "nulling interferometer" which makes use of on-axis destructive interferences to extinguish the stellar light while keeping the off-axis signal of the orbiting planet. Within the frame of the Darwin program, the European Space Agency (ESA) and the European Southern
Observatory (ESO) intend to build a ground-based technology demonstrator called GENIE (Ground based European Nulling Interferometry Experiment). Such a ground-based demonstrator built
around the Very Large Telescope Interferometer (VLTI) in Paranal will
test some of the key technologies required for the Darwin Infrared Space Interferometer. It will demonstrate that nulling interferometry can be achieved in a broad mid-IR band as a precursor to the next phase of the Darwin program. The instrument will operate in the L' band around 3.8 μm, where the thermal emission from the telescopes and the atmosphere is reduced. GENIE will be able to operate in two different configurations, i.e. either as a single Bracewell nulling interferometer or as a double-Bracewell nulling interferometer with an internal modulation scheme.
We are studying an optical concept aiming at recombining four mid-infrared telescope beams, where interference fringes are sampled in the pupil plane. Such a principle is perfectly adapted for reconstructing images by aperture synthesis with teh VLTI. It could be used for building a new generation 10 μm instrument, but instead of doing a totally new instrument, we propose the design of an optical module that can supply the surrent MIDI-VLTI instrument with 4 beams. The combined use of this module together with the MIDI instrument is the project called APreS-MIDI. Such an instrument at the VLTI focus will have an unique and very strong astrophysical potential.
APreS-MIDI (APerture Synthesis in the MID-Infrared) instrument function is to recombine 4 telescope beams of the VLTI. Interference fringes are sampled in the pupil plane. The optical principle uses "image densification". It is perfectly adapted for reconstructing images by aperture synthesis at 10mm. This principle could be used for building a new generation 10mm instrument, but instead of making a totally new instrument, we propose the design of an optical module that can supply the current MIDI-VLTI instrument with 4 beams.
The ARAL system of the VLTI is a multipurpose facility that helps to
have the interferometric instruments ready for night observations. It
consists of an artificial source (allowing a Mach-Zehnder mode of the
interferometric instruments for autotest), an alignment unit (verifying the position of the celestial target in the VLTI field-of-view), and an optical path router (controlling the optical switchyard and the instrument feeding-optics in the VLTI laboratory). With the multiplication of VLTI instruments and their specific features (wavelength coverage, number of beams), an upgrade of ARAL (from its November 2002 version) had to be carried out: the alignment unit has been redesigned, as well as the artificial source. This source will provide a point in the visible and in J, H, K and N infrared bands, split into four beams (with a zero optical path difference at the reference position). After a description of the optomechanics and of the computer architecture of ARAL, we detail the difficulties of building an interferometric artificial source with a wide spectral range.
In the last two years the Very Large Telescope Interferometer (VLTI) has been operated with a wavefront controlled down to the Coude focus of each 8m Unit Telescope. From this focus, the stellar beam is passively relayed by more than 10 mirrors distributed along a 100m subterranean path before to be coherently superimposed in the VLTI laboratory. Experience has proven that the observation efficiency would be largely improved by controlling the tilt of the beam directly inside the VLTI laboratory. In this article, we present the justification and basic features of the InfraRed Image Sensor (IRIS) as well as its implementation within the already packed VLTI laboratory. The forthcoming milestones of the project are presented.
The prime objective of GENIE (Ground-based European Nulling Interferometry Experiment) is to obtain experience with the design, construction and operation of an IR nulling interferometer, as a preparation for the DARWIN / TPF mission. In this context, the detection of a planet orbiting another star would provide an excellent demonstration of nulling interferometry. Doing this through the atmosphere, however, is a formidable task. In this paper we assess the prospects of detecting with nulling interferometry on ESO's VLTI, low-mass companions in orbit around their parent stars. With the GENIE science simulator (GENIEsim) we can model realistic detection scenarios for the GENIE instrument operating in the VLTI environment, and derive detailed requirements on control-loop performance, IR background subtraction and the accuracy of the photometry calibration. We analyse the technical feasibility of several scenarios for the detection of low-mass companions in the L'-band.
The Very Large Telescope Interferometer (VLTI) on Cerro Paranal (2635 m) in Northern Chile reached a major milestone in September 2003 when the mid infrared instrument MIDI was offered for scientific observations to the community. This was only nine months after MIDI had recorded first fringes. In the meantime, the near infrared instrument AMBER saw first fringes in March 2004, and it is planned to offer AMBER in September 2004.
The large number of subsystems that have been installed in the last two years - amongst them adaptive optics for the 8-m Unit Telescopes (UT), the first 1.8-m Auxiliary Telescope (AT), the fringe tracker FINITO and three more Delay Lines for a total of six, only to name the major ones - will be described in this article. We will also discuss the next steps of the VLTI mainly concerned with the dual feed system PRIMA and we will give an outlook to possible future extensions.
Two competitive design studies for the Ground-based European Nulling Interferometer Experiment (GENIE) have been initiated by the European Space Agency and the European Southern Observatory in November 2003. The GENIE instrument will most probably consist of a two-telescope Bracewell interferometer, using the 8-m Unit Telescopes and/or the 1.8-m Auxiliary Telescopes of the VLTI, and working in the infrared L' band (3.5 - 4.1 microns). A critical issue affecting the overall performance of the instrument is its capability to compensate for the phase and intensity fluctuations produced by the atmospheric turbulence. In this paper, we present the basic principles of phase and intensity control by means of real-time servo loops in the context of GENIE. We then propose a preliminary design for these servo loops and estimate their performance using GENIEsim, the science simulation software for the GENIE instrument.
Following the successful VLTI 'First Fringes' obtained in 2001 with the siderostats and with the 8m telescopes and based on the results from the commissioning phase, it is now possible to review with a critical eye the development approach followed over the last ten years and to draw a few conclusions.
We first recall this approach that aimed at minimizing the risk of not meeting the stringent requirements imposed by interferometry. This approach is based on the elaboration of exhaustive error budgets, an extensive set of analyses and early tests with feedback on subsystem specs, the performance characterization at subsystem level with identification of improvements when needed; and finally the commissioning of the complete VLTI at system level.
To illustrate this process, we provide practical examples taken from the project's history. We focus on two areas that have been considered among the most critical ones over the entire project life, namely: the turbulence inside the interferometer arms ('internal seeing') and the mechanical vibrations ('OPD stability').
For these two areas, we will finally compare the performances predicted during the development phase with those obtained at Paranal and we will draw conclusions.
Several scientific topics linked to the observation of extended structures around astrophysical sources (dust torus around AGN, disks around young stars, envelopes around AGBs) require imaging capability with milli-arcsecond spatial resolution. The current VLTI instruments, AMBER and MIDI, will provide in the coming months the
required high angular resolution, yet without actual imaging. As a rule of thumb, the image quality accessible with an optical interferometer is directly related to the number of telescopes used simultaneously: the more the apertures, the better and the faster the reconstruction of the image. We propose an instrument concept to
achieve interferometric combination of N telescopes (4 ≤ N ≤ 8) thanks to planar optics technology: 4 x 8-m telescopes in the short term and/or 8 x 1.8-m telescopes in the long term. The foreseen image reconstruction quality in the visible and/or in the near infrared will be equivalent to the one achieved with millimeter radio interferometers. Achievable spatial resolution will be better than the one foreseen with ALMA. This instrument would be able to acquire routinely 1 mas resolution images. A 13 to 20 magnitude sensitivity in spectral ranges from 0.6 to 2.5 μm is expected depending on the choice of the phase referencing guide source. High dynamic range, even on faint objects, is achievable thanks to the high accuracy provided by integrated optics
for visibility amplitude and phase measurements. Based on recent validations of integrated optics presented here an imaging instrument concept can be proposed. The results obtained using the VLTI facilities give a demonstration of the potential of the proposed technique.
On March 17, 2001, the VLT interferometer saw for the first time interferometric fringes on sky with its two test siderostats on a 16m baseline. Seven months later, on October 29, 2001, fringes were found with two of the four 8.2m Unit Telescopes (UTs), named Antu and Melipal, spanning a baseline of 102m. First shared risk science operations with VLTI will start in October 2002. The time between these milestones is used for further integration as well as for commissioning of the interferometer with the goal to understand all its characteristics and to optimize performance and observing procedures. In this article we will describe the various commissioning tasks carried out and present some results of our work.
The Very Large Telescope (VLT) Observatory on Cerro Paranal (2635 m) in Northern Chile is approaching completion. After the four 8-m Unit Telescopes (UT) individually saw first light in the last years, two of them were combined for the first time on October 30, 2001 to form a stellar interferometer, the VLT Interferometer. The remaining two UTs will be integrated into the interferometric array later this year. In this article, we will describe the subsystems of the VLTI and the planning for the following years.
Installed at the heart of the Very Large Telescope Interferometer (VLTI), VINCI combines coherently the infrared light coming from two telescopes. The first fringes were obtained in March 2001 with the VLTI test siderostats, and in October of the same year with the 8 meters Unit Telescopes (UTs). After more than one year of operation, it is now possible to evaluate its behavior and performances with a relatively long timescale. During this period, the technical downtime has been kept to a very low level. The most important parameters of the instrument (interferometric efficiency, mechanical stability,...) have been followed regularly, leading to a good understanding of its performances and characteristics. In addition to a large number of laboratory measurements, more than 3000 on-sky observations have been recorded, giving a precise knowledge of the behavior of the system under various conditions. We report in this paper the main characteristics of the VINCI instrument hardware and software. The differences between observations with the siderostats and the UTs are also briefly discussed.
In its current configuration, the VLT Interferometer (VLTI) combines the light collected by two telescopes and directs it towards the commissioning instrument called VINCI. In an interferometer, the optical path ranging from a telescope to the point where beams are combined is referred as an arm of the interferometer. This arm contains a large number of optics that have to be aligned at installation time and kept aligned during the period of use of the interferometer. The method used to perform the initial alignment is reported in a separate article. This paper is focussed on the methods used to assess the stability of the image alignment of each interferometer arm. Collected data sets are presented and interpreted.
With its `first fringe' milestone planned within a year the VLT Interferometer (VLTI) has entered a phase where it becomes possible to have a preliminary validation of its system performance by combining results of tests obtained at sub-system level. This is also the time for a careful planning of integration and commissioning activities. After an overview of the system approach used in the course of the VLTI project to predict and control the system performance, we present the results of the last crucial tests done on the thermal behavior of the underground, air-filled Delay Line Tunnel. These results have enabled to validate the early assumptions made on the effects of internal air turbulence (so called `internal seeing'). Finally, the last section presents the basic approach to the integration and commissioning of the VLTI to start this year at Paranal.
The Very Large Telescope (VLT) Observatory on Cerro Paranal (2635 m) in Northern Chile is approaching completion in this year when the fourth of the 8-m Unit Telescopes will see first light. At the same time, the preparation for first fringes of the VLT Interferometer (VLTI) is advancing rapidly with the goal of having the first fringes with two siderostats within this year. In this article we describe the status of the VLTI and its subsystems, we discuss the planning for first fringes with the different telescopes and instruments. Eventually, we present an outlook for the future of interferometry with Very Large Telescopes.
The pupil transfer, from the individual telescopes to the interferometric laboratory, is an unique feature of the VLT Interferometer allowing to have a 2 arcsec interferometric field available at the instruments entrance. This capability is the result of a careful analysis pursued from the very beginning of the VLTI until today in the interferometric laboratory layout. For this goal it has been necessary to develop a new optical device, the Variable Curvature Mirror (VCM), and also to design all the optical systems located after the delay-lines, as the beam compressors for instance, according to these interferometric field-of-view and pupil transfer requirements. This pupil transfer and the role/design of the various optical systems are presented for the major configurations of the VLTI. A special section is dedicated to the VCM system as this component is the most critical one and required special studies, using large deformation theory of elasticity, and advanced techniques in optical fabrication. The final performances of the VCM are reviewed. As these performances had an important influence ont he design of the other systems in the interferometric laboratory, the trade-off between the instruments requirements and the VCM capabilities is presented.
Star trails have been taken with a CCD-camera covering a field of approximately 10'. With the telescope movement stopped the sampling rate was of the order of 40 Hz. The correlations between the centroid variations of different trails, representing the image motion perpendicular to the trails, have been calculated. The power spectra of the centroid motions are used to identify their origin. The method turns out to be feasible for the measurement of the isoplanatic angle of image motion. The results show that the correlations of the image motions can be significantly enhanced, if the time delay due to the movement of the turbulence across the field of the telescope is taken into account.